This invention relates generally to television signal processing circuitry, and more particularly the invention relates to circuitry for ghost signal cancellation in television signals in all broadcasting standards.
Echo signals or ghost signals have been one of the major problems in modern television transmission. When a transmitted signal is received from the air, ghosts can be caused by reflections from mountains, buildings, and the like. In a television signal received through cable, the ghosts can be due to discontinuities of the connectors. Although the ghosts are generated in a radio frequency spectrum, the process in which the ghost is created can be accurately modelled as a linear distortion in a baseband signal. Therefore, cancelling the ghosts can be accomplished by passing the baseband signal through a linear filter which is an inverse of the linear model of the ghosting process.
Prior efforts in eliminating ghosts used analog technology such as a charge coupled device (CCD) transversal filter which did not produce significant improvement due to inaccuracy of the filter. More recently, a ghost canceller reference (GCR) signal has been proposed for use in training of filters. Digital filters have become widely accepted as alternative methods to implement the ghost cancellers.
Ghost signals can arrive at a receiver before and after the main signal, resulting in a precursor ghost and post-cursor ghost, respectively. A filter to cancel the precursor ghost is a physically non-realizable infinite impulse response (IIR) filter, which can be approximated by a longer finite impulse response (FIR) filter. The post-cursor ghost can be cancelled by an IIR filter. Therefore, the typical digital filter to cancel the ghost signals is an FIR feedforward filter followed by an IIR feedback filter.
In order to calculate the filter coefficients, a ghost canceller reference (GCR) signal is transmitted from the broadcasting station, typically in one line of the vertical blanking interval (VBI). The coefficient of the filters can be estimated by comparing the received GCR signal with a stored standard GCR signal. In order to cancel the unknown ghosts, the filters have to be able to adapt to different situations. Current methods of ghost cancellation are done by capturing the GCR data and then sending the data to a separate computing device, such as a DSP or microprocessing system, which then does calculations to determine the ghosts in the GCR data. The system then programs the coefficients of a set of filters for the video filtering.
The present invention is directed to a ghost cancellation system in which a signal processor or microprocessor is not required in updating coefficients in the feedforward and feedback filters of the active video ghost filter.